Some formation fluids, such as oils, contain a substantial amount of asphaltenes. Asphaltenes are large molecules that are dissolved within formation fluids at high pressures. As the pressure of the formation fluid is reduced, the solubility of the asphaltenes within the fluid is also reduced and the asphaltenes will begin to flocculate. The pressure at which the asphaltenes begin to flocculate is known as asphaltene onset pressure (AOP). FIG. 1 shows the process of flocculation. As shown in FIG. 1, individual molecules of asphaltenes form nanoaggregates and then form clusters of nanoaggregates.
In the oil and gas industry, the asphaltene onset pressure of a formation fluid within a hydrocarbon reservoir formation is valuable information that is used for completing and producing a well. For example, during production of a well, the formation fluid that is extracted from the hydrocarbon reservoir is maintained above the known asphaltene onset pressure to avoid creation of asphaltene clusters within the formation. A build-up of asphaltenes within the formation can curtail production of the well.
Using one technique, asphaltene onset pressure is measured in a laboratory environment by measuring light transmission through a large formation fluid sample (e.g., 10 mL-100 mL). The light transmission is measured while slowly reducing the pressure of the sample (e.g., 100 psi/hour). The sample is agitated using a mixer to maintain equilibrium within the formation fluid and to avoid asphaltene flocculation from settling within the cell. As the pressure within the formation fluid sample is decreased, at a certain pressure, the light transmission will decrease significantly. The pressure and temperature at which the light transmission will decrease significantly is the asphaltene onset pressure and the asphaltene onset temperature. FIG. 2 shows how flocculation of asphaltenes reduces light transmission. As the enclosed volume is increased, the pressure of the oil sample decreases and asphaltenes begin to flocculate at the asphaltene onset pressure. At this point, the formation fluid sample turns opaque and reduces the transmission of light. An intense light source, such as a laser is used for such light transmission measurements. Another laboratory technique also uses a slow depressurization technique, but identifies asphaltene flocculation using microscopic observation. In yet another laboratory technique, the light transmission technique and the microscopic observation technique are combined so that light transmission and direct observation of flocculation are performed simultaneously.
While these slow depressurization techniques can be used in a laboratory environment, the techniques are not well suited for measurement of asphaltene onset pressure in a wellbore environment because the techniques use (i) a large formation fluid sample, (ii) additional mixing equipment, (iii) a large timescale for depressurization, and (iv) a bright light source, such as a laser.
Another factor that complicates measurement of asphaltene onset pressure is that asphaltene onset pressure can be confused with bubble point pressure. The bubble point pressure is the pressure at which at least a portion of a liquid changes phase to a vapor state (e.g., nucleates bubbles) at equilibrium. FIG. 3 shows a formation fluid sample in a liquid state within an enclosed volume. As the size of the enclosed volume is increased, the pressure of the formation fluid sample decreases and bubbles begin to form at the bubble point pressure. This formation of bubbles will also reduce the light transmission of a formation fluid sample. The decrease in light transmission occurs because bubbles form at the bubble point and the bubbles scatter light, which reduces light transmission. Accordingly, asphaltene onset pressure can be confused with bubble point pressure because both the asphaltene onset pressure and the bubble point pressure reduce transmission of light.